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250 Directional phased vertical antennas
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J1 antenna A J3 transmitter J2 antenna B L1A L1B L1C
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pass or insert a phase-shifting length of coaxial cable (L4) For 180 phasing use the following equation to find the length (L4): L= where L is the length of L4, in feet VF is the velocity factor (a decimal fraction) FMHz is the operating frequency, in megahertz Some people use a series of switches to select varying amounts of phasing shift from 45 to 270 Such a switch allows them to select any number of other patterns for special situations 492 VF FMHz ft [111]
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The phased vertical antenna concept can be used to provide round-the-compass control of the antenna pattern Figure 11-5A shows how three quarter-wavelength verticals (arranged in a triangle that is a half-wavelength on each side) can be used to provide either end-fire or broadside patterns from any pair (A-B, A-C, or B-C) Any given antenna (A, B, or C) will be grounded, fed at 0 , or fed with 180 The table in Fig 11-5B shows the relative phasing for each direction that was labelled in Fig 11-5A Either manual phase changing or switch-operated phase changing can be used, although the latter is preferred for convenience Some international showcase broadcasters use antenna arrays formed into two or more concentric circles of vertical elements, with one element at the center Selection of elements and phasing determines directivity and gain
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252 Directional phased vertical antennas
A L4 B
11-4 Phase-shifting antenna circuit
360 directional array 253
NW2 N NE1
NW1 C
W A B SE1 SW2 SE2 S SW1 11-5A Three-element phased array
254 Directional phased vertical antennas
Ant A 0 0 GND 0 0 GND Ant B 0 GND 0 180 GND 0 Ant C GND 180 0 GND 0 180 11-5B Table of feed phasing for the three-element array Ant = antenna GND = ground
Direction N-S NE1-SW1 NE2-SW2 E-W SE1-NW1 SE2-NW2
CHAPTER
Directional beam antennas
THE DIRECTIONAL BEAM ANTENNA DOES SEVERAL JOBS FIRST, IT PROVIDES AN APPARENT
increase in radiated power, because it focuses available transmitter power into a single (or at worst limited) direction For this reason, a bidirectional dipole has a gain of approximately 2 dB over an isotropic radiator Add one or more additional elements, and the focusing becomes nearly unidirectional, which increases the effective radiated power (ERP) even more Second, the beam increases the received signal available at the inputs of the receiver Antennas are generally reciprocal, so they will work for receiving as they do for transmitting Finally, the directivity of the beam antenna allows the operator to null interfering stations In fact, it is the last attribute of the beam that is most useful on today s crowded bands All in all, if your funds are too little to provide both increased RF power and a good antenna system, then spend what is available on the antenna not on the power In this chapter we will focus on directional antennas that can be built relatively easily It is assumed that most readers who want a triband multielement Yagi will prefer to buy a commercial product, rather than build a homebrew model The material herein concentrates on homebrew projects that are within the reach and capabilities of most readers The first of these is not a beam antenna at all, but rather a rotatable dipole
Rotatable dipole
The dipole is a bidirectional antenna with a figure-8 pattern (when viewed from above) The dipole is a half-wavelength and is usually installed horizontally, although vertical half-wavelength dipoles are known Although the length of the dipole is too great for rotatability at the lower bands, it is within reason for the higher band For example, the size of the halfwave dipole is approximately 16 ft on 10 m and 22 ft on 15 m Even the 33-ft length on 20 m is not unreasonable for amateur constructors The length of the dipole is found from
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256 Directional beam antennas 468 FMHz This length is approximate because of end effects and other phenomena, so some cut and try is required Example 12-1 Find the length of a dipole antenna for a frequency of 24930 MHz in the 12-m amateur radio band Solution: 468 L ft : FMHz 468 24930 MHz 1877 ft
[121]
The half-wave dipole is fed in the center by coaxial cable Each element of the dipole is one-half of the overall length (or, in the example given, about 94 ft) Figure 12-1 shows a rotatable dipole that can be designed for use on 15, 12, and 10 m The radiator elements are made from 10-ft lengths of 3 4-in aluminum tubing The tubing is mounted on beehive standoff insulators, which in turn are mounted on a 4-ft length of 2 2 lumber The lumber should be varnished against weathering In a real pinch, the elements can be mounted directly to the lumber without the insulators, but this is not the recommended practice The mast is attached to the 2 2 lumber through any of several means The preferred method is the use of a 1-in pipe flange These devices are available at hardware stores under the names floor flange and right-angle flange The 10-ft lengths of pipe are the standard lengths available in hardware stores, so it was selected as being closest to the required 22 ft for 15 m A 014- H loading coil is used at the center, between the elements, in order to make up for the short length The dimensions of the coil are 4 to 5 turns, 05-in diameter, 4-in length For low power levels, the coil can be made of no 10 (or no 12) solid wire and, for higher levels, 1 8-in copper tubing There are two basic ways to feed the antenna, and these are shown in details A and B in Fig 12-1 The traditional method is to connect the coaxial cable (in parallel) across the inductor This method is shown in Fig 12-1, detail A A second method is to link couple the coil to the line through a one- to three-turn loop (as needed for impedance matching) This is the method that would be used for a toroidal inductor Lower frequencies can be accommodated by changing the dimensions of the coil The coil cannot be scaled, simply because the relative length of the antenna changes as the frequency changes But it is possible to cut and try by adding turns to the coil, one turn at a time, and remeasuring the resonant frequency Adding inductance to the coil will make the antenna usable on 17 m and 20 m, as well as on 15 m Another method for building a rotatable dipole for lower frequencies is to increase the element lengths On 17 m, the overall length is approximately 274 ft, so each element length is 137 ft long This length can be achieved by either of two methods First, adjacent sizes of aluminum tubing are designed so that the smaller will be a slip-fit inside of the larger What constitutes adjacent sizes depends on the wall thickness, but for one common brand, the 7 8-in is adjacent to the 3 4-in size You
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